1. Field of the Invention
The present invention relates generally to smartphones, and more particularly but not exclusively to methods and apparatus for calibrating vehicle and smartphone coordinate systems.
2. Description of the Background Art
Smartphones are mobile telephones that include computing resources for data processing and data communications. Some smartphones further include various sensors, such as a 3-axis accelerometer and a 3-axis magnetic field sensor, and have an integrated global positioning system (GPS). Examples of currently-available smartphones include the APPLE IPHONE and ANDROID smartphones.
Smartphone sensors may be employed in a variety of vehicle-related applications, such as driving habits detection, car accident detection, and the like. Because the orientation of a smartphone relative to a vehicle varies depending on how the user (driver or passenger) of the vehicle carries the smartphone, these vehicle-related applications require calibration of the coordinate system of the smartphone to that of the vehicle. More particularly, information from smartphone sensors is relative to the smartphone, not the vehicle. Therefore, to understand the movement of the vehicle, information from smartphone sensors need to be translated in terms of the vehicle's coordinate system.
One calibration method involves mounting the smartphone at a fixed position that is aligned with the vehicle's coordinate system. Although this calibration method is relatively simple, it is inconvenient to the user as the smartphone needs to be manually mounted and dismounted. This method also prevents or makes it difficult to use the smartphone for other purposes.
Another method of calibrating the coordinate systems of the smartphone and the vehicle requires the use of a GPS and a 3-axis magnetic field sensor. In this method, the smartphone's coordinate system is transferred to the world's coordinate system using sensor information from the smartphone's accelerometer and the 3-axis magnetic field sensor. The world's coordinate system is then transferred to the vehicle's coordinate system using GPS information. Although this method works independent of the orientation of the smartphone, not all smartphones are equipped with a GPS. Furthermore, GPS increases power consumption of the smartphone, is not available in certain locations, and may be inaccurate depending on the smartphone.